1. Field of the Invention
The present invention relates to a chuck for chucking a cutting tool such as a drill, and more particularly to a chuck in which a clamp sleeve is fitted onto the outer circumference of a chuck sleeve of a holder body via needle rollers, and in which, through rotation of the clamp sleeve in a clamping direction, the diameter of the chuck sleeve is reduced in order to tightly chuck a desired tool.
2. Description of the Related Art
A conventional chuck will be described with reference to FIGS. 8 and 9. FIG. 8 is a cross-sectional view of a tool chuck, and FIG. 9 is an exploded perspective view of the tool chuck, showing the structural components thereof.
As shown in FIGS. 8 and 9, the tool chuck has a holder body 1. The tool holder body 1 has a tapered shank portion 1A to be inserted into a spindle of an unillustrated machine tool, a gripping flange 1B formed at the larger-diameter end of the tapered shank portion 1A, and a chuck sleeve 1C that extends from the end surface of the flange 1B opposite the tapered shank portion 1A. The chuck sleeve 1C extends in the direction away from the tapered shank portion 1A such that the axis of the chuck sleeve 1C coincides with the axis of the tapered shank portion 1A. A tapered surface 1Ca is formed on the outer circumference of the chuck sleeve 1C such that the diameter of the surface 1Ca decreases toward the tip end thereof.
Numeral 2 denotes a roller retaining sleeve which is fitted onto the outer circumference of the chuck sleeve 1C with a clearance therebetween. The diameter of the roller retaining sleeve 2 decreases toward the tip end thereof at the same rate as that of the tapered surface 1Ca of the chuck sleeve 1C. The roller retaining sleeve 2 is held by a retainer ring 3 provided on the outer circumference of the tip end portion of the chuck sleeve 1C such that the roller retaining sleeve 2 is prevented from coming off the chuck sleeve 1C.
In the roller retaining sleeve 2, numerous needle rollers 4 are disposed to form a plurality of rows. The needle rollers are fitted into the roller retaining sleeve 2 in such a manner that the needle rollers incline in the circumferential direction at a predetermined angle with respect to the center axis of the roller retaining sleeve 2. Each of the needle rollers 4 has a diameter greater than the wall thickness of the roller retaining sleeve 2, so that the portion of each needle roller 4 projecting inward from the inner circumferential surface of the roller retaining sleeve 2 is in contact with the tapered surface 1Ca of the chuck sleeve 1C, while the portion of each needle roller 4 projecting outward from the outer circumferential surface of the roller retaining sleeve 2 is in contact with the inner circumferential surface of a clamp sleeve 5, which will be described below.
The clamp sleeve 5 is adapted to decrease the diameter of the chuck sleeve 1C to thereby tightly chuck a desired tool. The clamp sleeve 5 is fitted onto the outer circumference of the chuck sleeve 1C via the needle rollers 4 held by the roller retaining sleeve 2. The clamp sleeve 5 has a tapered inner circumferential surface whose diameter decreases from the end facing the flange 1B toward the tip end thereof. Further, a sealing/retaining ring 6 is attached to the inner circumference of the base end portion of the clamp sleeve 5 adjacent to the flange 1B. The sealing/retaining ring 6 is in contact with the outer circumferential surface of the chuck sleeve 1C so as to provide a sealing function. The sealing/retaining ring 6 also abuts the end surface of the roller retaining sleeve 2 in order to prevent the clamp sleeve 5 from coming off the chuck sleeve 1C.
In the conventional chuck having the above-described structure, when a tool 7 is to be chucked, a spring collet 8 is fitted onto the shank 7A of the tool 7, and the shank 7A of the tool 7, together with the spring collet 8, is inserted into the cylindrical bore of the chuck sleeve 1C. When the clamp sleeve 5 is rotated clockwise, each needle roller 4 in contact with the tapered inner surface of the clamp sleeve 5 revolves spirally along the outer surface of the chuck sleeve 1C while rotating about its own axis. Thus, the roller retaining sleeve 2 moves toward the flange 1B, while rotating, in accordance with the revolution of the needle rollers 4, and at the same time the clamp sleeve 5 also moves toward the flange 1B. The movements of the roller retaining sleeve 2 and the clamp sleeve 5 toward the flange 1B reduce the clearance between the tapered inner surface of the clamp sleeve 5 and the tapered outer surface of the chuck sleeve 1C, so that the clamp sleeve 5 strongly presses the chuck sleeve 1C from the entire outer circumference thereof via the needle rollers 4 in order to reduce the diameter of the chuck sleeve 1C. As a result, the shank 7A of the tool 7 inserted into the cylindrical bore of the chuck sleeve 1C is clamped and held by the chuck sleeve 1C via the spring collet 8.
When the clamp sleeve 5 in a clamped state is rotated counterclockwise, the roller retaining sleeve 2 moves toward the tip end of the chuck sleeve 1C, and the clamp sleeve 5 also moves in the same direction. Therefore, the diameter of the chuck sleeve 1C is restored to its original size, so that the clamping force applied to the tool 7 is released. This enables the tool 7 to be removed from the chuck sleeve 1C or be replaced with a different tool.
In such a conventional chuck, the chuck sleeve 1C extends from the end surface of the flange 1B opposite the tapered shank portion 1A. Therefore, within an area 1Ca adjacent to the base portion of the chuck sleeve 1C continuous with the flange 1B, the rigidity against elastic deformation in the radial direction is considerably higher than that in the remaining area. Therefore, when the clamp sleeve 5 is rotated in the clamping direction so as to reduce the diameter of the chuck sleeve 1C, within the above-described area 1Ca the ratio of reduction in diameter is very small compared to that in the remaining area. This phenomenon will be described with reference to FIG. 2.
FIG. 2A is a graph showing variation in the internal diameter of the conventional chuck sleeve 1C in a clamped state, and FIG. 2B shows a cross sectional view of the conventional chuck sleeve 1C.
When the conventional chuck sleeve 1C shown in FIG. 2B is clamped by the clamp sleeve 5 and the reduced internal diameter is measured at each of points P1 to P12 situated from the open end of the chuck sleeve 1C to the flange 1B, the reduction in internal diameter changes as shown by a curve 62 in FIG. 2A. As is apparent from the curve 62, the reduction in internal diameter is relatively large and constant in an area corresponding to the points P2 to P9, whereas in the area 1Ca corresponding to the points P10 to P12 the reduction in internal diameter becomes very small and the rate of the reduction increases towards the flange 1B.
The above-described phenomenon occurs because of elastic deformation of the clamp sleeve 5. That is, in the area 1Ca corresponding to the points P10 to P12, the rigidity against radial elastic deformation increases with decreasing distance from the flange 1B, and therefore the portion of the clamp sleeve 5 facing the area 1Ca deforms elastically to increase its diameter. As a result, the chucking force in the area 1Ca decreases, with the result that the shank 7a of the tool 7 cannot be held with a uniform chucking force over the entire length thereof. Moreover, the effective chucking length L1 over which the chuck sleeve 1C actually chucks the tool 7 decreases.
Especially, when heavy cutting is conducted through use of a tool that is chucked over such a short effective chucking length L1, there occurs an eccentric motion in which the tip end of the chuck sleeve 1C serves as a fulcrum. As a result, in the area 1Ca where the chuck force becomes low, friction is generated between the outer peripheral surface of the tool shank and the inner surface of the chuck sleeve 1C, resulting in damage to the outer peripheral surface of the tool shank and the inner surface of the chuck sleeve 1C.
When the clamp sleeve 5 is rotated in the clamping direction until the clamp sleeve 5 abuts the flange 1B of the holder body 1, the reduction in internal diameter within the area 1Ca becomes large to some degree. However, in this case, a large clamp force must be applied to the clamp sleeve 5. Further, within the area 1Ca the load acting on the needle rollers becomes excessively large, and in worst case, in addition to the surface of the needle rollers, the outer peripheral surface of the chuck sleeve 1C and the inner surface of the clamp sleeve 5 which are in contact with the needle rollers are damaged, resulting in a decrease in the force of chucking a tool.
Moreover, when the length of the chuck sleeve 1C is increased in order to increase the effective clamping length L1, a so-called tool length L2 of the holder body 1 increases, and therefore the size and weight of the chuck increase.